Switching mode power supplies (SMPS) are widely used in electronic equipment due to their high efficiency. Switching frequency of a SMPS is a crucial parameter in designing the power supply since it affects the size of all other components such as switching devices, inductors, capacitors, etc. One switching frequency control method is fixed frequency control. For fixed frequency control, the switching frequency remains constant for all operating conditions. For step-down voltage regulators (e.g., buck voltage regulators), in order to achieve a high output voltage close to its input voltage, a low switching frequency is used to increase duty cycle. However, for a wide input voltage range, when operating at a high input voltage level, the low switching frequency will result in high inductor current ripple. Furthermore, when operating at a high input voltage level, low operation efficiency and/or other performance degradations can occur. For example, a notebook computer power system can receive a 19V input voltage source during an adaptor power mode and a 5.6V input voltage source during a battery power mode. Furthermore, the adaptor power mode and the battery power mode can both be required to provide an output voltage of 5V in order to power a hard disk or USB device. It is therefore desirable for the SMPS to operate at lower switching frequency at low input voltage in order to achieve a large duty cycle, and to increase the switching frequency at higher input voltage to maintain performance of circuits in the SMPS.
However, SMPS are typically designed with a constant switching frequency in order to optimize components for an intended application. For example, with a fixed switching frequency for a SMPS, different inductor values are used for different output voltage levels. In a high volume production environment, a manufacturer is thus required to stock different components (e.g., different inductors for a filter), increasing component management cost and/or component maintenance cost.